Peripheral vagal control of heart rate is impaired in neuronal NOS knockout mice

Abstract

The role of nitric oxide (NO) in the vagal control of heart rate (HR) is controversial. We investigated the cholinergic regulation of HR in isolated atrial preparations with an intact right vagus nerve from wild-type (nNOS+/+, n = 81) and neuronal NO synthase (nNOS) knockout (nNOS-/-, n = 43) mice. nNOS was immunofluorescently colocalized within choline-acetyltransferase-positive neurons in nNOS+/+ atria. The rate of decline in HR during vagal nerve stimulation (VNS, 3 and 5 Hz) was slower in nNOS-/- compared with nNOS+/+ atria in vitro (P < 0.01). There was no difference between the HR responses to carbamylcholine in nNOS+/+ and nNOS-/- atria. Selective nNOS inhibitors, vinyl-L-niohydrochloride or 1-2-trifluoromethylphenyl imidazole, or the guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one significantly (P < 0.05) attenuated the decrease in HR with VNS at 3 Hz in nNOS+/+ atria. NOS inhibition had no effect in nNOS-/- atria during VNS. In all atria, the NO donor sodium nitroprusside significantly enhanced the magnitude of the vagal-induced bradycardia, showing the downstream intracellular pathways activated by NO were intact. These results suggest that neuronal NO facilitates vagally induced bradycardia via a presynaptic modulation of neurotransmission.

Cardiac enkephalins attenuate vagal bradycardia: interactions with NOS-1-cGMP systems in canine sinoatrial node

“…This finding does not rule out a similar role for NOS in other parts of the neuroeffector pathway such as the intracardiac parasympathetic ganglion (29). However, the present findings are consistent with reports that have localized neuronal NOS in vagal nerve terminals innervating the mouse and rat SA node and the guinea pig atria (7,22). Neither MEAP or the NOS inhibitors appear to interact with cardiac pacemaker cells because all three agents were ineffective when the bradycardia was produced by the intranodal delivery of the direct acting muscarinic agonist methacholine.…”

“…If this difference is real, another NOS isoform, perhaps endothelial NOS (eNOS) also may have contributed to the overall response. However, immunocytochemical approaches failed to demonstrate eNOS in vagal nerve terminals in the SA node (7,22) and when eNOS was knocked out, vagal bradycardia was unaltered (7).…”

“…The NOS-cGMP system's influence may be limited under control conditions because none of the NOS intermediates increased baseline neurotransmission and all were effective only when transmission was first suppressed by enkephalin or NOS inhibitors (7,17,18). Within the NOS pathway, NOS may be a point of limitation because bypassing NOS restored vagal transmission faster than adding the NOS substrate L-arginine.…”

“…A selective neuronal NOS (NOS-1) inhibitor produced a qualitatively similar vagolytic effect suggesting that the affected enzyme was resident within the network of intrinsic and extrinsic cardiac nerves (18). NOS-1 was immunocytochemically localized in choline acetyltransferase-positive cells within the atria and NOS inhibitors were ineffective when the mixed cholinergic agonist carbachol was substituted for direct nerve stimulation (7,18). These findings suggested that NO improves vagal control of heart rate by facilitating the local release of acetylcholine.…”

The Pharmacology of Nitric Oxide in the Peripheral Nervous System of Blood Vessels

“…Neuronal NO was suggested to facilitate vagally induced bradycardia via a prejunctional modulation of neurotransmission (Herring et al, 2000). This hypothesis was supported by studies with nNOS knockout mice (Choate et al, 2001). …”